Thin, one-piece spade bits, such as that shown in U.S. Pat. No. 4,682,917 "Williams", are effective for boring holes into wood only up to 11/2 inch diameter. These spade bits include a central axial projection, which serves as a pilot, and forward edges which serve as cutting edges. In order to bore larger holes in the range of up to 3 inch diameter, a heavier cutting head is needed.
Self-feeding wood bits with heavier cutting heads are known in the art. Those bits have a pilot point with threads which pull the bit into the wood. They perform better than spade bits, because they cut faster and require little axial force. With a heavier cutting head, it becomes quite difficult and expensive to form the bit as a single piece. Therefore, the tendency in the art has been to make the bit in several pieces and then fasten them together by various means. Often, the cutting head is welded or otherwise joined to the shaft, and the piloting point is inserted into a square or other non-circular hole and held in place with a set screw.
Of course, as might be expected, the piloting points tend to work loose from the rest of the bit. Also, the cost of welding pieces together is a substantial part of the cost of manufacturing these bits.
A bit which does not require welding is shown in U.s. Pat. No. 2,593,823 "Wilson". That bit is intended for lighter duty than the present invention. In the Wilson patent, the cutting head of the bit is very short, which means that there is little thread engagement between the shaft and the cutting head, causing a tendency for instability of the head relative to the shaft. In that threaded arrangement, most of the forces between the cutting head and the shaft are borne in a very small area near the shoulder, thereby tending to cause failure of one or both parts at that point. Also, because the cutting head is so short, there will tend to be instability between the cutting head and the workpiece.